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 In 1909 the physicist Ernest Rutherford directed an experiment at the University of Manchester in England to measure small deflection angles recently observed when alpha particles--tiny positively charged bodies given off by radioactive elements--were beamed through a thin gold foil. He wanted to gauge the distribution and charge of matter within the atom. The then-current theory held that the atom was like a pudding, a diffuse, positively charged sphere studded with negatively charged electrons.
As illustrated here, the setup included a radioactive source, a target consisting of a thin sheet of gold foil, and a detector consisting of a screen covered with zinc sulfide. Although atoms and subatomic particles are much too small to be seen directly, particles hitting the screen would leave microscopic marks in the zinc sulfide. The pattern of marks in the screen (opposite) was a huge surprise and could not be accounted for by the "pudding" atom. In 1911 Rutherford proposed an explanation: The atom was largely empty space, so most of the tiny alpha particles could pass unimpeded. But in the center of the atom was a minuscule and highly charged nucleus that held most of the atomic mass--something like the Sun in the center of the solar system. Alpha particles deflected by the nucleus bounced back in unexpected ways.
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  Expected Results
High-energy alpha particles should pass through a thin gold foil only a few atoms thick (left), leaving a small region at the back of the zinc screen covered with dots (right). |
 Unexpected Results As predicted, dots appeared mostly at the back of the screen (right), but every so often dots were scattered near the front of the screen, as if they had ricocheted. |
A radioactive source (below) aimed a stream of alpha particles at a very thin gold foil surrounded by a screen covered with zinc sulfide (left) |  |
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New Atomic Theory
Rutherford's explanation was that the ricocheting alpha particles had bounced off something small, dense, and positively charged in the center of the atom (above). The new model of the atom—largely empty space with a compact nucleus—both explained the surprising results of the experiment and was a milestone in modern atomic theory. |
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also to distant planets, stars, and galaxies. Consequently, physics experiments have led to a "standard model" for the nature of matter and forces throughout the universe. The basic constituents of matter in this standard model are not protons or neutrons, but quarks (page 75), the fancifully named specks first proposed in 1964 by the American physicists Murray Gell-Mann and George Zweig. In the end, quarks may not prove to be nature's ultimate building blocks, but for now they explain all the properties of matter we have observed. Most physicists think that an overarching theory will supersede the standard model, just as relativity superseded Newton's laws of motion. Such a theory would combine all known forces and particles into one elegant recipe of the cosmos.
Any good theory of nature must account for the properties of elements, the fundamental substances that cannot be broken down into something simpler. The ancient Greeks thought there were four elements: fire, air, water, and earth. They also proposed a fifth element: the essence of the heavens, called "quintessence." Today, we know of more than 115 elements, about 90 of which exist naturally. (Scientists forge the rest in their labs, but most of them exist only for fractions of a second before they decay radioactively into other elements.) Think back to your childhood, when you may have played with Tinkertoys or other sets of interconnecting pieces. You created a seemingly endless array of contraptions using perhaps a dozen different shapes and sizes of building blocks. Now, imagine such a game with 90 varieties of pieces. That's the flexibility nature has at its disposal to construct our universe.
However, nature cannot mix elements in a haphazard way. Certain rules dictate which elements can combine. For instance, "noble gases" such as helium and neon almost never react with anything, just as a noble lord might ignore the common folk in his domain. Fluorine and chlorine, on the other hand, grab hold of just about anything (continued)
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